DR. CECIL H. FOX / SCIENCE PHOTO LIBRARY DR. CECIL H. FOX / SCIENCE PHOTO LIBRARY
Becke's lines around a crystal of sodium chloride in polarized light. The white line surrounding the crystal faces shows that the refractive index of the crystal (1.50) is lower than the surrounding refraction oil (1.5150). In 1903 Frederich Becke (1855-1931) presented a major work on the composition and texture of crystalline schists. He described a simple technique for determining the refractive index of minerals by immersing tiny samples of them in liquids with known refractive indices and observing them with an uncomplicated compound microscope, with or without polarized light. If the refractive index of the mineral was different, lines would appear around the specimen. If the medium was more refractive than the specimen (ie if the refractive index of specimen was higher than the mounting medium) a black halo could be seen at the edge as the microscope was moved out of focus. When the refractive index of the specimen was lower than the medium, a white line could be seen around the specimen. If the refractive indices of both were the same, no lines could be seen.
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